1. Field of the Invention
The present invention relates to a driving mechanism in which an actuator is used, and, in particular, to a driving mechanism for driving an optical member such as a relatively small-sized lens to be mounted, for example, on a small digital camera, a web camera and a camera-equipped cellular phone.
2. Description of the Related Art
A piezoelectric element-used actuator is known as a driving mechanism for a lens used in a digital camera and the like. For example, the actuator disclosed in JP-A-2002-142470 is constituted by a piezoelectric element, a driving member and a base. A driving member is bonded to the one end surface of the piezoelectric element in the elongating and contracting direction, and a driven member is frictionally engaged with the driving member. Further, a base is bonded to the other end surface of the piezoelectric element in the elongating and contracting direction. The above-mentioned constitution makes it possible to transmit a movement of a piezoelectric element in the elongating and contracting direction to the driving member when a pulse voltage is applied to the piezoelectric element. Where the piezoelectric element deforms gradually, the driven member moves together with the driving member. Where the piezoelectric element deforms at a great speed, the driven member remains at the same position due to inertia of the mass. Accordingly, application of a pulse voltage is repeated, thereby allowing the driven member to move intermittently at a fine pitch.
The thus constituted actuator may pose a problem that the actuator resonates between a base, piezoelectric element and driving member. Further, in order to suppress the resonance effect on attachment to a body, in the actuator disclosed in JP-A-2002-142470, the base is bonded to the body by use of a rubber-based adhesive agent, thereby supporting the actuator to the body in an elastic manner. However, this driving method is extremely difficult in controlling a constitutional variation in the base, the piezoelectric element and the driving member.
Utilization of the above-described resonance is advantageous in that movement amount of the driving member is increased. An actuator utilizing resonance has been disclosed, for example, in Japanese Patent No. 3171187. According to the actuator, a pulse voltage is applied in accordance with displacement of a piezoelectric element on resonance thereby increasing a displacement amount of a driven member.
However, in the actuators disclosed in JP-A-2002-142470 and Japanese Patent No. 3171187, when an attempt is made to utilize a resonance state which occurs inside the actuator constituted by a piezoelectric element, driving member and base, a problem occurs that the driving member is affected by the resonance and displaced in a direction other than the elongating and contracting direction of the piezoelectric element. For example, as shown in FIG. 27A and FIG. 27B, a problem occurs that the driving member 2 is affected by the resonance and displaced in a direction other than the elongating and contracting direction of the piezoelectric element. Therefore, a driving force generated by the elongation and contraction of the piezoelectric element 1 is not accurately transmitted to a driven shaft 14, thereby making it difficult to correctly move the driven shaft 14 in the elongating and contracting direction of the piezoelectric element 1.
In order to solve the above-described problems, it is necessary to support the actuator to a body (fixed frame) so that the actuator is less affected by resonance. A method for supporting an actuator is proposed, for example, in JP-A-2002-95274, in which the actuator is supported by use of a driving shaft or a driven member other than a piezoelectric element.
However, in the actuator disclosed in JP-A-2002-95274, a driving shaft is attached to one side of a piezoelectric element, and while the other side is free. Therefore, a problem occurs that when a high-frequency voltage is applied to the piezoelectric element to elongate and contract, a problem occurs that the free other side is greatly displaced but the driving shaft side is hardly displaced to fail in gaining a sufficient driving force of a driven member. Therefore, it is necessary to attach a weight member to the other side of the piezoelectric element. In this case, as described above, a problem occurs that a resonance is generated between the piezoelectric element, the weight member and the driving shaft, thereby making it impossible to accurately move the driven member.